1. Field of the Invention
The present invention relates to a coupling circuit of a transmission line to a dielectric resonator.
2. Description of the Related Art
A prior art TE.sub.01.delta. mode dielectric resonator employed in a bandpass filter and the method of coupling with its external circuit are shown in FIG. 1 through FIG. 3. In FIG. 1, between two standard waveguides (i.e. TE.sub.10 mode waveguides) 1 and 1' there is connected a second waveguide 2 which is in a cut-off state for the electromagnetic wave to be now transmitted through the standard waveguides 1 and 1'. A TE.sub.01.delta. mode cylindrical dielectric resonator element 3 is installed in the second waveguide 2 via a metal stage 4 mounted on its side wall parallel to the larger side walls of the standard waveguides 1 and 1'. The resonator element 3 is coupled magnetically, i.e. via magnetic flux, with both the standard waveguides 1 and 1', so as to allow only the resonator element's resonant frequency to transmit through the cut-off waveguide 2. In this circuit configuration, the stage 4 causes an increase in space occupancy of the circuit.
In order to reduce the space occupancy, a configuration shown in FIG. 2 has been proposed, such as disclosed in Japanese TokuKai Hei-1-144701. In this circuit configuration, a half-cut cylindrical dielectric resonator element 5 has its flat surface adhered to a shorter side wall of the cut-off waveguide 2, and is magnetically coupled with the standard waveguides 1 and 1'.
In FIG. 3, a half-cut dielectric resonator element 5 is adhered on an inner wall of a metal case 7 so as to interconnect coaxial lines 6 and 6'. In this circuit configuration, an extension of each of the inner conductors of the coaxial lines 6 and 6' is terminated on the metal case 7 and forms a loop 6a which is magnetically coupled with the half-cut cylindrical resonator element 5.
However, there are problems in that in the FIG. 2 configuration the overall circuit size is little reduced even though the resonator element is reduced into a half size; and in the FIG. 3 configuration the loops 6a require the space in the case 7. The same problem is in a circuit configuration employing a quarter cut TE.sub.01.delta. -mode dielectric resonator element reported in "IEEE Transaction on Microwave Theory and Techniques", vol. MTT-35, No. 12, December 1987, p.1150-1155. Thus, there is no much likelihood of further size reduction in the above-described circuit configuration. Therefore, a new coupling circuit which can enjoy the advantage of the compact half or quarter cut cylindrical dielectric resonator has been expected.